Bidirectional Shift Register

In the field of digital electronics and computer science, shift registers play a vital role in storing and manipulating data. Among the different types of shift registers, the bidirectional shift register holds a prominent position. This article aims to provide a comprehensive understanding of bidirectional shift registers, including their structure, working principle, and applications.

Structure of a Bidirectional Shift Register

A bidirectional shift register is a sequential circuit that can shift data both leftwards and rightwards. It consists of a series of flip-flops connected in a chain, allowing the data to be shifted in either direction. The number of flip-flops used determines the size of the shift register and its data storage capacity.

Bidirectional Shift Register Circuit Diagram

A bidirectional shift register is a versatile circuit used for shifting data either to the right or left based on an input signal. It can be implemented using D flip-flops and logic gates, enabling the transfer of data from one stage to the next stage in the desired direction determined by a mode control signal. This flexible functionality makes bidirectional shift registers an essential component in various digital systems.

Working Principle of Bidirectional Shift Register

The bidirectional shift register operates based on clock pulses and control signals. It has two modes of operation: shift right and shift left. Let’s explore these modes in detail:

1. Shift Right: In the shift right mode, the data is shifted from the leftmost bit to the rightmost bit. Each clock pulse triggers the transfer of data to the adjacent flip-flop on the right. The rightmost bit receives the input data, while the leftmost bit is discarded.
2. Shift Left: In the shift left mode, the data is shifted from the rightmost bit to the leftmost bit. Each clock pulse triggers the transfer of data to the adjacent flip-flop on the left. The leftmost bit receives the input data, while the rightmost bit is discarded.

Bidirectional shift registers often have additional control inputs for shifting operations, such as parallel load, asynchronous clear, and synchronous clear. These inputs enable various functionalities, such as loading data into the register, clearing the register, and synchronizing the register with external events.

Applications of Bidirectional Shift Registers

Bidirectional shift registers find applications in various domains, including digital communication systems, data storage, arithmetic operations, and control systems. Here are some notable applications:

1. Serial-to-Parallel Conversion: Bidirectional shift registers can convert serial data streams into parallel data, enabling efficient data processing. By shifting the data into parallel registers, it becomes easier to manipulate and analyze the information.
2. Parallel-to-Serial Conversion: Conversely, bidirectional shift registers can also convert parallel data into serial data. This functionality is useful for transmitting data over serial communication channels or for interfacing parallel devices with serial devices.
3. Data Storage and Delay: Bidirectional shift registers can serve as temporary storage devices, allowing data to be stored and retrieved at a later time. They are widely used in applications that require delay elements or buffer stages.
4. Arithmetic Operations: Bidirectional shift registers are essential components in arithmetic circuits, such as multiplication and division units. They facilitate the shifting and manipulation of binary numbers, enabling efficient arithmetic operations.
5. Control Systems: In control systems, bidirectional shift registers are employed to implement shift and rotate operations on binary control signals. These operations are vital for sequential logic and state machine designs.

Let us see some difference between Unidirectional and Bidirectional Shift Register:

Features	Unidirectional Shift Register	Bidirectional Shift Register
Data Movement	A unidirectional shift register can shift data in only one direction, either left or right.	A bidirectional shift register can shift data in both the left and right directions.
Structure	It consists of a series of flip-flops connected in a chain, with data flowing in a single direction.	It also consists of a series of flip-flops connected in a chain, but it allows data to flow in both directions.
Control	Unidirectional shift registers typically have a single control input to determine the direction of data shifting.	Bidirectional shift registers have control inputs or modes that determine the direction of data shifting.

Example:

Implement a 4-bit bidirectional shift register and perform the following operations:

1. Load the initial data of 1011.
2. Shift the data to the right twice.
3. Shift the data to the left once.
4. Load new data of 1100.
5. Shift the data to the right once.
6. Shift the data to the left twice.

Solution

Step 1. Load the initial data of 1011:

Load:    1 0 1 1
Shift:   X X X X
Output:  1 0 1 1

Step 2. Shift the data to the right twice:

This step involves shifting the data to the right twice. With each clock pulse, the data in the shift register moves one position to the right. The rightmost bit is discarded, and the input data is shifted in from the left. In this case, the initial data of 1011 is shifted twice to the right, resulting in the output Q being 0101.

Load:   X X X X
Shift:  0 1 0 1
Output: 0 1 0 1

Step 3. Shift the data to the left once:

In this step, the data is shifted to the left once. Similar to the previous step, the data moves one position to the left with each clock pulse. The leftmost bit is discarded, and the input data is shifted in from the right. The previous output Q of 0101 is shifted once to the left, resulting in the output Q being 1010.

Load:   X X X X
Shift:  1 0 1 0
Output: 1 0 1 0

Step 4. Load new data of 1100:

Load:   1 1 0 0
Shift:  X X X X
Output: 1 1 0 0

Step 5. Shift the data to the right once:

This step involves shifting the newly loaded data to the right once. The data moves one position to the right with each clock pulse, and the input data is shifted in from the left. The new data of 1100 is shifted once to the right, resulting in the output Q being 0110.

Load:   X X X X
Shift:  0 1 1 0
Output: 0 1 1 0

Step 6. Shift the data to the left twice:

In this final step, the data is shifted to the left twice. Similar to the previous steps, the data moves one position to the left with each clock pulse, and the input data is shifted in from the right. The previous output Q of 0110 is shifted twice to the left, resulting in the output Q being 1000.

Load:   X X X X
Shift:  1 0 0 0
Output: 1 0 0 0